Multiband orthogonal frequency division multiplexing (MB-OFDM) is a modulation technique used in some wireless communication systems such as ultra-wideband (UWB). The MB-OFDM modulation technique combines OFDM modulation with frequency hopping. It is a modulation technique suitable for devices designed to comply with Federal Communications Commission (FCC) regulations relating to UWB devices. According to the current regulations, UWB devices are allowed to operate in the frequency band between 3.1 to 10.6 GHz, provided that the bandwidth occupied by a UWB device is at least 500 MHz and the radiated power, when measured over a bandwidth of 1 MHz, is less than −41.3 dBm anywhere within the signal band.
In some MB-OFDM systems, encoded bits are transmitted as OFDM symbols that each includes Quadrature Phase Shift Keying (QPSK) modulated sub-carriers (also referred to as tones) and pilot tones. Typically, an OFDM symbol includes a set of sub-carriers provided via a local oscillator (LO). In some embodiments, after the OFDM symbol is transmitted, the LO switches its frequency setting and a different set of sub-carriers are provided for the next symbol. The resulting waveform is a frequency hopping waveform in which each OFDM symbol corresponds to a hop in the transmit frequency band. In some embodiments, the LO remains at the same frequency setting throughout the transmission.
FIG. 1 is a diagram illustrating a frequency hopping pattern according to some MB-OFDM system embodiments. In this example, each rectangle represents an OFDM symbol carrying 200 bits. Three hop bands f1, f2 and f3 are used to facilitate frequency hopping between symbols. Symbols 100, 102 and 104 are each modulated using a different set of sub-carriers associated with frequency bands f1, f2 and f3, respectively. The pattern is then repeated for the next three symbols, 106, 108 and 110 and so on. In this example, the OFDM symbol rate is 3.2 Mbps, giving an over-the-air bit rate of 200*3.2=640 Mbps.
Although frequency hopping is useful for reducing interference among adjacent UWB devices, several issues remain. When data is transmitted at a rate lower than the maximum rate supported by the system, data bits are commonly duplicated and then modulated for transmission. The same data bit may be modulated multiple times on to the same sub-carrier frequency, causing the duplicated symbols to be subject to the same multipath and fading conditions and resulting in reduced frequency diversity. Furthermore, adjacent data bits are sometimes encoded using sub-carriers that are the same or close in frequency, also resulting in reduced frequency diversity. It would be desirable if the data bits could be encoded such that frequency diversity of the same data bits as well as adjacent data bits could be improved. It would also be desirable if the encoding scheme could be implemented without adding significant additional cost and complexity to the existing transmitter and receiver designs.